This invention relates to a sliding surface bearing for heavy loads, comprising a backing shell and a bearing metal layer provided on said backing shell and formed with axially spaced apart grooves, which extend at an acute angle of at most 15 degrees to the peripheral direction substantially around the axis of the bearing and contain a bearing material which is softer than said bearing layer.
The requirements to be met by sliding surface bearing are in part conflicting. For instance, good emergency running properties include a low resistance to plastic deformation but this is inconsistent with a high fatigue strength at operating temperatures. In an endeavor to utilize the advantages afforded by different bearing materials while avoiding their disadvantages, sliding surface bearings for heavy loads usually comprise a plurality of layers. A backing shell of steel carries a layer of an aluminum- or copper-base bearing alloy. The bearing alloy layer carries a bearing surface layer of a bearing material, such as white metal or plastic, which is softer than the bearing alloy. That bearing surface layer is supported by the harder bearing alloy layer so that the disadvantages of the several materials used in the bearing cannot become fully effective. But the effects of the undesired properties of these materials cannot be entirely suppressed. A softer bearing surface layer can never have the same fatigue strength and wear resistance as a bearing surface consisting of a harder bearing material, even if the softer bearing surface layer is supported by a harder intermediate layer.
In an endeavor to reduce friction and to increase wear resistance, it has already been proposed in German Early Disclosure 22 51 637 to provide a bearing surface which is not formed by a continuous layer of a single bearing material but comprises a softer bearing material, which is embedded in a layer of a harder bearing metal so that zones of harder and softer bearing materials alternate in the peripheral direction. That proposal has been based on the concept that the harder bearing metal constitutes a carrying skeleton, which will prevent an excessive flow of the softer bearing material to the rubbing surfaces. Because the protruding lands of the carrying skeleton extend transversely to the peripheral direction they may become coated with a thin layer of the softer bearing material. Such sliding surface bearings have not performed as expected, whether ther softer bearing material consisting preferably of a self-lubricating plastic material was contained in axial grooves or in helical grooves formed in the harder bearing material. Surprisingly it has been found that the tendency of the softer bearing material to be spread by friction cannot by considerably reduced.
In another known sliding surface bearing of that kind, described in French Patent Specification No. 797,483, the grooves which receive the softer bearing material are helical and extend at an acute angle to the peripheral direction rather than substantially at right angles thereto so that grinding cuts will be performed in the peripheral direction at the transitions from the softer to the harder bearing material and solid particles disposed at a transition from a softer bearing material to the harder can be forced only to a very small extent from the softer into the harder material but will mainly remain embedded in the softer material, where they will not give rise to local overloads. But these known sliding surface bearings have a relatively low load-carrying capacity.
It is an object of the invention to avoid the above-mentioned disadvantages of the known sliding surface bearings and to provide a sliding surface bearing which is suitable for heavy loads and affords the advantages of softer and harder bearing materials in combination but is free from the disadvantages of such materials.
In a sliding surface bearing of the kind described first hereinbefore this object is accomplished according to the invention in that the axial center spacing of the grooves does not exceed an upper limit EQU a.sub.o =250+0.5d+001d.sup.2
where a.sub.o is expressed in micrometers and d is the diameter of the bearing surface in millimeters, and said center spacing in any case does not exceed 10 mm.
It has surprisingly been found that a sliding surface bearing for heavy loads in which said condition is fulfilled will meet all requirements. The conditions resulting from a local overload should depend not only on the properties of one or the other of the bearing materials but on a combination of said properties. For this purpose a sufficiently fine distribution of the softer bearing material over the bearing surface must be ensured. That distribution will depend on the spacing of the grooves, which will be selected in dependence on the area of the surface which carries load at a time, i.e., on the diameter of the bearing surface. A sufficiently fine distribution will be ensured if the spacing is below the above-mentioned upper limit a.sub.o, although the groove spacing should not exceed 10 mm even in bearings which are large in diameter.
As it is essential to combine the effects of the softer and harder bearing materials and said combined effect will depend on the distribution of the bearing materials on the bearing surface, the results can be improved by a finer distribution. Particularly good results will be obtained if the axial center spacing of the grooves is below an upper limit EQU a.sub.o =200+0.3d+0.001d.sup.2
where a.sub.o is expressed in micrometers and d is the diameter of the bearing surface in millimeters.
There is also a lower limit to the axial center spacing of the grooves, not only with a view to the manufacture but also to ensure that a sufficiently large volume of the softer bearing material will be available in grooves having a desirable cross-sectional shape. If the axial center spacing of the grooves is larger than or equal to a lower limit EQU a.sub.u =25 o 0.1d
where a.sub.u is expressed in micrometers and d is the diameter of the bearing surface in millimeters, all requirements in that respect can be desirably met.
It has been stated hereinbefore that the risk that hard solid particles may become embedded in the harder bearing material depends on the angle between the grooves and the peripheral direction. That risk will be virtually eliminated if the acute angle between the grooves and the peripheral direction is not in excess of 3 degrees. That requirement can be met in a simple manner because the desired small angle will be obtained be necessity if the grooves are helical and spaced apart as stated. If the grooves include small acute angles with the peripheral direction, a disturbance of the hydrodynamic film of lubricant will not be disturbed at the transitions between the harder and softer bearing materials by boundary zones which extend at right angles to the peripheral direction.
Owing to the loads applied and the desired strength, the softer bearing material should be distributed in a desirable manner over the bearing surface and the grooves should have a desirable cross-sectional shape. That cross-sectional shape can be defined by the ratio of the depth t of the grooves to the axial center spacing a of the grooves. That ratio should not exceed an upper limit 0.5 (1500-d)/1500 and should not be less than one-fifth of said upper limit, where d is the diameter of the bearing surface in millimeters. It has been found in practice that particularly desirable results will be obtained if said ratio does not exceed an upper limit of 0.2 (1500-d)/1500. Regardless of the diameter of the bearing surface, that ratio should not be less than 0.03.